Plug current regulator

ABSTRACT

A plug current regulator for a dc electric traction motor propelled vehicle, implemented using a microcomputer for controlling a time-ratio switching regulator, provides a substantially smooth electrical braking function by applying relatively short time duration current pulses to build up braking current, followed by a relatively long time duration in which current pulses are inhibited. Current regulation is provided by comparing motor current to a desired braking current and generating an update signal when actual current drops below desired current. Filtering is minimized by sampling motor current at a time interval occurring relatively long after a current pulse is supplied to the motor.

This application is a continuation, of application Ser. No. 333,928,filed Dec. 23, 1981 now abandoned.

Reference is made to microfiche appendix which sets forth a computerprogram listing including that which is applicable to the presentinvention. Included are 1 microfiche containing a total of 39 frames.Cross reference is made to related application Ser. No. 331,931"Electric Vehicle Current Regulator", assigned to General ElectricCompany and filed currently herewith.

BACKGROUND OF THE INVENTION

The present invention relates to dc electric motor current regulatorsand, more particularly, to a method and apparatus for regulatingelectrical braking current in a dc electric traction motor.

In present day electric vehicles, electronic power regulators are usedto control the torque, or speed, developed by the electric tractionmotors. Typically, the regulator comprises a time-ratio or choppercircuit which varies the power developed by the motors by controllingthe percentage of time that the motors are connected directly to a powersource. For maximum mobility, the power source is a battery, whichlimits the available power to the motors. The regulator also includesapparatus responsive to accelerator position for varying the mark-spaceratio of the chopper circuit.

In order to reduce the rate of wear of mechanical brakes in electricvehicles such as forklifts or other industrial trucks, it is commonpractice to implement some form of electrical braking. A common form ofelectrical braking is dynamic braking or plugging in which the motorarmature is short-circuted by a diode and motor field current, andtherefore, torque is regulated by the chopper circuit. In general, lowlevel of field current generates a relatively high magnitude of armaturecurrent. Regulation to a desired braking torque under these conditionstends to be inefficient since the armature current is so large withrespect to field current that armature reaction disturbs the normalfield flux control of armature current. Because such a low level offield current excitation at higher armature velocities produces verylarge magnitudes of armature current, the control of initiation ofplugging becomes relatively critical.

When a chopper circuit is used to control motor current, the transitionfrom a motoring mode to a braking mode typically requires that thechopper circuit switch from a relatively high percent on-time to arelatively low percent on-time. A typical transition might be from a 95percent on-time per cycle to a one percent on-time per cycle. The lowmark-space ratio at plug initialization sometimes caused cogging orjerking due to double pulsing, a condition resulting from one currentpulse bringing the plug current almost to a desired level followed by asecond pulse forcing a large current overshoot. Part of the reason fordouble pulsing comes from the fact that the control loops are slightlyunstable due to a large time delay between armature current and machineflux. The chopper applies voltage to the field winding while armaturecurrent is being monitored. By the time armature current reaches thedesired value, field current, and therefore machine flux, has increasedabove the level necessary to generate the desired armature current thuscausing a torque overshoot. To some extent, the plug current regulatingproblem stemmed from the need to be able to use the same current sensorin both motoring and braking modes of operation. The typical currentsensor filtering to provide an indication of average current and inelectric vehicle applications might respond to a point at about 80percent up on the ripple curve, i.e., the typical approach is to attemptto regulate the current peaks. In these applications, transient voltagespikes resulting from the chopping action in the inductive circuit havehigh values which affect the accuracy of the control system.

It is an object of the present invention to provide an improved currentregulating system for electrical brakes of a dc electric motor.

It is another object of the invention to provide a plug currentregulating system for a dc electric motor switching regulator controlsystem which reduces jerking torque on the motor.

It is another object of the invention to provide a plug currentregulating system for a dc electric motor switching regulator controlsystem which avoids double or multiple pulsing of the motor.

It is a further object of the invention to provide a plug currentregulating system for a dc electric motor switching regulator controlsystem which responds to absolute values of armature current withoutfiltering.

SUMMARY OF THE INVENTION

A current regulator is implemented in a microcomputer control system inwhich a switching regulator operating in a time-ratio control moderegulates current to a dc electric traction motor. When electricalbraking is desired, the control system terminates gating signals to theswitching regulator and automatically transitions to the braking mode inwhich a switching regulator operating in a time-ratio control moderegulates current to a dc electric traction motor. When electricalbraking is desired, the control system terminates gating signals to theswitching regulator and automatically transitions to the braking mode inwhich the time-ratio of conducting to non-conducting time is immediatelyset at a very low percent. Current regulation is thereafter achieved byonly responding to armature current readings taken at a predeterminedtime interval after termination of the conducting time of the regulator.Preferably, the control system includes a comparator for comparingdesired braking current to actual braking current, which comparator isused, after the predetermined time, to initiate the next conducting timeinterval of the regulator when actual current falls below desiredcurrent. In this arrangement, the system regulates on the valleys of thebraking current waveform thus eliminating the phase lag problems causedby inductive transients.

DESCRIPTION OF THE DRAWING

The novel features which are believed to be characteristic of theinvention are set forth in the appended claims. The invention itself,however, both as to its advantages and objects thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawing in which:

FIG. 1 is a simplified schematic diagram of DC electric motor powercircuit;

FIG. 2 is a graphical representation of the torque/speed characteristicsof a DC motor with constant voltage excitation;

FIG. 3 is a graphical representation of the torque/speed characteristicsof a DC motor with constant current excitation;

FIG. 4 is a functional block diagram of a DC motor control system inaccordance with the concepts of the present invention;

FIG. 5 is a graphical representation of the torque/speed characteristicsof a DC motor controlled in accordance with the present invention;

FIG. 6 is a simplified block diagram of a microcomputer implementedcontrol system in accordance with the present invention.

FIG. 7 is an expanded block diagram of the microcomputer system of FIG.6;

FIGS. 8-15 represent flow diagrams for software implementation of thecontrol system of FIG. 7.

FIG. 9 is a graphical representation of the operation of the disclosedsystem in a plugging mode.

DETAILED DESCRIPTION

The principal elements in a power circuit of a battery powered directcurrent series motor control system are shown in FIG. 1. A battery 10 isconnected to a positive power bus 11 through contactor tips 12. A timeratio control power modulating circuit (chopper) 14 connects bus 11 to afield winding 16 and an armature 18 of a series motor. A flyback diode17 connected across the series combination of field 16 and armature 18provides a motor current path when chopper 14 is non-conductive. Inoperation the main line contactor tips 12 are closed and the powersupplied to the series motor is regulated by the time ratio ormark-space ratio of the electronic chopper 14. In an electric vehicleapplication, the operator must control the time ratio output of thechopper 14 through position of an accelerator pedal (not shown) and relyupon the relationship between the accelerator pedal position and markspace ratio to control vehicle torque or speed.

The characteristic curves for a typical traction motor are illustratedin FIG. 2 where the abscissa is torque and the ordinate is speed. Thecurves 20, 22, 24 and 26 represent the torque/speed relationship of aseries traction motor with varying levels of applied voltage. Curve 26represents the characteristic for high value of applied voltage andcurve 20 represents the characteristic for a lower level of appliedvoltage. In a typical traction motor application, such as a batterypowered forklift truck which operates at speeds so low that wind is anegligible factor, the torque required to move the truck at any velocityis independent of velocity and based only upon the rolling frictioncoefficient, the tires, the gross weight of the vehicle and the gearratio. Characteristic curve 28 is typical of the load characteristicrequired to drive such a vehicle at various speeds. Since for a givenvalue of battery potential, the voltage applied to the motor is simply aproduct of the percent conduction time of chopper 14 and batterypotential, then characteristic curves 20, 22, 24, and 26 could also beavailable as direct functions of the chopper mark-space ratio. In theusual electric vehicle application, the accelerator pedal position istranslated directly into mark-space ratio, i.e., 100 percent pedal wouldcorrespond to 100 percent conduction time, 50 percent pedal wouldcorrespond to 50 percent conduction time and 10 percent depression ofthe accelerator pedal would correspond to 10 percent conduction time.Thus, the balancing speed, which is represented by the intersection ofthe load curve 28 and percent on-time speed torque characteristics 20,22, 24 and 26 each corresponding to a particular pedal depression,determines the speed of the motor. With full pedal depression, the motoraccelerates up to the maximum speed determined by the intersection ofcurves 26 and 28. With a lesser pedal depression corresponding tocharacteristic curves 24, 22 or 20, the speed is correspondingly less.It is therefore possible to regulate the speed of the vehicle. However,note at very low speed one obtains very high torque for a very smallpedal depression. For example, in characteristic curve 24, whichpossibly corresponds to only 30 or 40 percent pedal depression, oneobtains a torque far in excess of that required to move a vehicle. Inaddition, in practice the electronic chopper 14 is limited in maximumcurrent by a current protection system and, in a typical system, 30percent battery voltage supplied to the motor is sufficient to reach themaximum current level capability of the electronic chopper. Thus atclose to zero speed, all of the torque control would be confined to thefirst 30 percent of accelerator pedal depression thereby tending to makethe pedal very sensitive over that range and unresponsive over 70percent of its range.

Other systems for controlling the torque in an electric vehicle makemotor current rather than voltage, directly proportional to acceleratorpedal position. The characteristics one would obtain from such a controlare illustrated in FIG. 3 where the abscissa is torque and the ordinateagain is speed. The outer envelope 26 is the motor torque/speed curvewith maximum applied voltage. Characteristic curves 30, 32, 34, 36 and38 are the torque speed curves at five different levels of constantmotor current. Considering, for example, the operation on characteristiccurve number 30, which might correspond to a 20 percent depression ofthe accelerator pedal, the system would provide 20 percent of themaximum capability of the chopper 14 at a very low voltage. As the motorspeed increases, it would continue to operate on this same curve movingvertically away from the torque axis with the motor terminal voltageincreasing as speed increases to counterbalance the effect of the motorgenerated EMF and maintain the voltage difference necessary to keep thatlevel of current flowing through the armature and field resistances.Eventually, as the speed increased, it would reach a point where fullbattery voltage must be applied to the motor in order to maintain thatdesired level of current. This point corresponds to the intersection ofthe characteristic curve 30 on the outer envelope 26. The curvedesignated 48 represents the characteristic load curve for a heavytraction type vehicle where windage is not a factor. It will beappreciated that the current regulated type of control system has verygood performance at low speed since any increase in pedal depressionfrom 0 to 100 percent transitions along the torque axis throughcharacteristic curves 30, 32, 34, 36 and 38, where 38 corresponds to themaximum current capability of the electronic chopper 14. Thus, the fullcurrent, i.e., torque, capability of chopper 14 corresponds to fullpedal depression of the accelerator. However, since the loadcharacteristic curve 48 is parallel to the pedal characteristic curves30, 32, 34, 36 and 38, for a given weight of vehicle there is no onegiven pedal position which will maintain any fixed speed but ratherthere will be one pedal position which exactly balances the tractiveeffort necessary to maintain a constant speed of the vehicle.

For example, if balancing torque occurs at 75 percent pedal depressionwhen the vehicle is traveling at 5 miles per hour, 75 percent pedaldepression will also maintain vehicle speed at 10 miles per hour. Thus,in the upper speed region, accelerator pedal sensitivity is similar tothat in the voltage control system at lower speeds providing pooroperator "feel".

Referring now to FIG. 4 there is shown a functional block diagram of aregulator system according to the present invention. An accelerator 50provides a current reference signal on line 52 and a voltage referencesignal on line 68. The signal on line 52 is a current reference which isdirectly proportional to accelerator depression, i.e., 100 percentaccelerator depression provides 100 percent output in current referencewhich corresponds to the maximum capability of the power source. Thesignal on line 52 is coupled to an input terminal of a current limitcircuit 54 which may reduce the current reference signal in the event ofan over-temperature or other monitored condition. The signal output ofcurrent limit circuit 54 is coupled via line 56 to a controlacceleration circuit 58 which limits the rate of increase of the currentreference signal from a present value to a new value. The signal outputfrom the control acceleration circuit 58 is coupled via line 60 to aninput terminal of a summing junction 64. The signal on line 60 iscompared to the measured motor current in summing junction 64.

A filter circuit 66 has an input terminal connected to an outputterminal of junction 64. Filter circuit 66 limits the rate of increaseof the signal from junction 64. Connected to the filter circuit 64 is alimit circuit 70 which is designed to restrict the signal output of thefilter circuit 64 on a percentage basis to be equal to or less than thepercent pedal depression corresponding to the voltage reference signalon line 68. In other words the signal output from filter circuit 66cannot exceed the percent of its range represented by the percent ofaccelerator pedal depression, i.e., if the accelerator pedal isdepressed 50 percent, then the signal from filter circuit 66 cannotexceed 50 percent of its maximum value. The limit circuit 70 iscontrolled by the voltage reference signal on line 68 from accelerator50.

The signal from circuit 66 is provided via line 71 to an oscillator 72whose mark-space ratio corresponds to the percent on-time represented bythe signal on line 71 i.e., when the signal on line 71 attains 50percent of its maximum value, the oscillator 72 provides a 50 percentmark-space ratio output signal. The signal from oscillator 72 is coupledto a power amplifier 74 which regulates current in motor 76. Motorcurrent is sensed and a signal representative thereof is coupled vialine 62 to summing junction 64.

The regulator system outlined in FIG. 4 is thus comprised of a referencegenerating section including accelerator 50, current limit circuit 54and control acceleration circuit 58 which generates a current referencesignal on line 60 proportional to pedal depression with maximum currentlimits subject to the power and temperature limits of the power unit.The rate at which the current reference can be increased is constrainedby the control acceleration circuit 58 to a constant rate and since thetorque of a dc traction motor is proportional to current, the rate oftorque increase is a constant. This tends to provide a betterresponsiveness independent of speed. The pedal depression also generatesa motor voltage limit reference signal which is proportional to pedaldepression to limit the maximum motor voltage to a value proportional topedal depression. A current regulating loop comprised of filter circuit66 which responds to the difference between the current reference signalon line 60 and the measured motor current signal on line 62 foradjusting the oscillator 72, and power amplifier 74 regulate current inmotor 76. The effect of voltage limit circuit 70 is to shift the controlmode from current to voltage regulation to improve the speed control athigher speeds.

The operation of the regulator of FIG. 4 may be better evaluated byreference to FIG. 5 where torque is once again the abscissa and speedthe ordinate. The transition line 98 delineates the division of thetorque-speed plane between voltage and current regulation. The verticallines 82, 84, 86, and 88 represent the constant current outputs at 100,75, 50 and 25 percent pedal depression, respectively. The characteristiclines 80, 90, 92 and 94 represent the constant voltage torque/speedcurves at 100, 75, 50 and 25 percent of battery voltage respectively.The vertical dashed line 96 is the required load torque representing thetractive effort necessary to overcome the rolling friction for a givenvehicle weight. If the vehicle was initially assumed to be at rest, andspeed assumed to be equal to zero and the pedal depressed 25 percent,corresponding to the current line 88, the vehicle would not move sincethe developed torque would be less than the load torque. Should thepedal be depressed to 50 percent corresponding to the current line 86,the developed torque would exceed the load torque and the vehicle wouldbegin to move. As the speed increases, the motor terminal voltage willalso increase to the value necessary to maintain 50 percent current.When a speed is reached which corresponds to the intersection ofcharacteristic line 86 with characteristic line 98, the motor terminalvoltage will reach 50 percent battery voltage corresponding to amarkspace ratio of 50 percent. The voltage limit circuit 70 will thenlimit the mark-space ratio to 50 percent forcing continued accelerationto follow the constant motor voltage line 92 to the intersection withload torque line 96. The intersection point is referred to as thebalancing speed point since the developed torque exactly balances theload torque. If the accelerator pedal is then depressed completely, thetorque will increase along the line 100, with the rate of torqueincrease dictated by the control acceleration circuit 58, until themark-space ratio reaches 100 percent or full battery voltage is appliedto the motor terminals. The vehicle will accelerate at a rateproportional to the differe between the lines 80 and 96 until the nearbalancing speed represented by the intersection between lines 80 and 96is reached. If the accelerator pedal is then relaxed and againcompletely depressed, the motor torque will first be reduced to zero andthereafter increased to the value corresponding to the intersection oflines 80 and 96 with the rate of increase determined by the controlacceleration circuit 58.

Although the functional block diagram of FIG. 4 is believed sufficientto enable those skilled in the art to make and use the presentinvention, reference is now made to FIG. 6 wherein there is illustrateda preferred implementation of the invention using a microprocessor basedarrangement. The basic power circuit for the DC motor comprisingarmature 18 and field winding 16 includes a variable mark-space ratiochopper circuit 14 and a key switch 12 which serve to connect the motoracross the battery 10. Preferably chopper circuit 14 comprises a siliconcontrolled rectifier (SCR) chopper circuit including a controllablecommutation circuit and associated commutating capacitor. A typicalchopper is shown in U.S. Pat. No. 3,826,959 issued July 30, 1974 andassigned to General Electric Company. The free-wheeling diode 17provides a path for inductive current when chopper circuit 14 switchesto a non-conductive state. A plugging diode 102 connected in reverseparallel with armature 18 provides a reverse current path to preventself-excitation during electrical braking.

The field winding 16 is arranged to be connected in either a forward orreverse direction in series with armature 18, where forward and reverserefer to the direction of rotation of the motor armature 18, by means ofcontacts F1, F2 and R1, R2. The contacts F1, F2, R1 and R2 are shown intheir normal de-energized state. Control of contacts F1 and F2 isthrough a contactor actuating coil 104 while contacts R1 and R2 arecontrolled by a contactor actuating coil 106. The coils 104 and 106 areconnected across the battery 10 by means of respective contactor drivercircuits 108 and 110. The driver circuits 108 and 110 may be in the formillustrated in co-pending application Ser. No. 299,047 filed Sept. 3,1981 and assigned to the General Electric Company.

The control functions are implemented in a central processing unit (CPU)112 which includes the necessary hardware such as counters, registers,memory units and microprocessors for performing those functionsdescribed in FIG. 4. The CPU 112 is connected to perform selected safetychecks by monitoring the status of a seat switch 114, a brake switch 116and forward and reverse direction switches 118 and 120. The accelerator50 also provides an input signal indicative of the percent acceleratorpedal depression to the CPU 112. Motor armature current sensing isprovided by a sensor 122 connected in series with armature 18.

Referring now to FIG. 7, the CPU 112 is shown in more detail. Amicrocomputer 124 is coupled via an address bus 126 and a data bus 128to a plurality of input/output interfaces. The microcomputer 124preferably comprises a type 6502 microprocessor available from RockwellInternational Corp., a type 74LSI38 address decoder available from TexasInstruments, Inc., and addressable random access memory (RAM) and readonly memory (ROM) sufficient for program storage and storage ofintermediate and computed or monitored variables. Intel Corp. types 2114and 2716 are suitable for RAM and ROM, respectively.

A first signal conditioning circuit 130 is connected to receive thearmature current signal from sensor 122. The signal conditioning circuit130 adjusts the amplitude of input signals to a level compatible withthe apparatus connected to its output terminals, in this instance ananalog to digital (A/D) converter 132. A/D converter 132 may be, forexample, a type ADC0816 available from National Semiconductors, Inc. Thedigitized output signals from A/D converter 132 are coupled onto theaddress and data busses 126 and 128 under control of the microcomputer124. In addition to the armature current signal, the accelerator pedalposition signal and various current limit signals are also coupled tothe microcomputer 124 through signal conditioning circuit 130 and A/Dconverter 132.

A second signal conditioning circuit 134 provides an interface betweenmicrocomputer 124 and those system signals which are of a binary nature,i.e., those signals representative of switches being open or closed orof the system being in a propulsion or braking mode of operation. Aninput/output (I/O) interface circuit 136 couples the signals from signalconditioning circuit 134 to the address and data busses 126 and 128. TheI/O circuit may be, for example, a type 6522 available from RockwellInternational Corp. As illustrated, the signal conditioning circuit 134monitors the status of seat switch 114, brake switch 116, forwarddirection switch 118 and reverse direction switch 120. A plug signaldeveloped by a comparator 138 is also monitored by circuit 134. The plugsignal is provided during electrical braking and switches between afirst state when motor armature current is greater than the desiredmagnitude of plug current and a second state when motor armature is lessthan the desired magnitude of plug current.

Command signals developed by the microcomputer 124 are coupled throughan I/O interface 140 and signal conditioner 142 to the control devices,e.g., the forward/reverse contactor driver circuits 108 and 110 and theswitching devices within chopper circuit 14. The I/O interface 140 mayalso be a Rockwell International type 6522 device. The signalconditioning circuit 142 is a driver amplifier and level shiftingcircuit of a type well known in the art.

Also connected to the address and data busses 126 and 128 is a digitalto analog (D/A) converter circuit 144 whose function is to provide ananalog output signal representative of the desired magnitude of brakingcurrent during electrical braking or "plugging." The D/A circuit 144 maybe a type AD558 available from Analog Devices, Inc. The signal from D/Acircuit 144 is coupled to an input terminal of comparator 138 forcomparison to the actual motor current signal. It will be noted that themotor current signal is conditioned or scaled in signal conditioningcircuit 130 before being coupled via line 146 to an input terminal ofcomparator 138. Functionally, the microcomputer implemented regulatorcircuit of FIGS. 6 and 7 operates substantially as shown in thefunctional block diagram of FIG. 4. Appendix A represents the softwareprogram for implementing the functions described in the functional blockdiagrams. Referring now to FIGS. 8 et seq., there is shown a moreprecise flow diagram of the current regulator function. In the motoringmode the regulator routine is entered at REGLN 150. Whenever acontactor, such as the forward or reverse contactors, is energized, itis desirable to inhibit the regulator function in order to avoidoscillations which might be caused by contact tip bouncing. Theregulator routine includes a provision for setting a tip bounce timerduring the first pass through the routine. During the inhibit interval,typically about twenty milliseconds, only current update functions arepermitted, i.e., no gating pulses to the chopper 14 are generated.During each pass, the routine checks the status of a tip bounce flag todetermine if the timer has timed out. If the tip bounce flag is setindicating that the timer has not timed out, the routine goes into asecondary loop at REGL13 to be described infra. If the tip bounce flagis not set the routine checks to determine if the drive motor iscurrently energized, i.e., if chopper circuit 14 has been operating, bychecking the status of a flag (initial turn on flag) which is set whengating signals are supplied to chopper circuit 14. If the motor has notbeen energized, the routine branches into a zero set mode and sets thecurrent reference to zero or an initialized condition and then returnsto the main routine.

Since the normal current limit function is distinct from the plugcurrent limit function, the routine checks whether the system is in anelectrical braking, i.e., plug, mode or a propulsion mode. If the plugflag is set, the routine branches into a plug regulation mode (PREG) tobe described infra. In the propulsion mode, motor current is read at thesame time during each chopping cycle of chopper circuit 14. A flag (MTR.CUR. READ) is set during that time to inhibit regulator operation for atime period, approximately 100 microseconds, sufficient to allow thereading to be taken and settled before it is sampled. The flag iscleared at the end of each reading cycle. When the routine detects thatthe flag has been cleared, thus indicating that a new reading isavailable, the routine sets the READ flag for the next cycle. If theREAD flag is set when this point in the routine is reached, the routineis exited (REGXIT) for the present cycle.

MOTOR CURRENT<0 is intended to eliminate amplifier drifts at or nearzero motor current. Since motor current should always be a positivevalue, this step checks that value and, if it is positive, saves thevalue for future use. If negative, motor current is set to zero andsaved.

Beginning at FIG. 9 a motor current reference value (TOTAL REF.) equalto the sum of the accelerator pedal reading plus a creep reference iscompared against a maximum current limit. The pedal reading may belimited, as previously described, by the controlled acceleration orcurrent limit functions. The creep reference is a small offset referencevalue intended to keep strain on gearing in the vehicle without anypedal depression so as to minimize gear slap at start-up. If the motorcurrent reference value is greater than the maximum permitted value,e.g., 800 amperes, the reference is set to 800 and the routinecontinues. If the reference value is less than 800 amperes, the routineproceeds using the actual value.

Several comparisons next occur for determining whether the desired motorcurrent, i.e., the reference value, exceeds various current limitfunctions. One step compares the reference value against a commutationlimit reference (CLREF) which varies as a function of percent on time ofchopper circuit 14. Another step compares the reference value against apredetermined maximum current limit (CL) value. The smaller of the CLREFor CL determines the magnitude of error signal in the event that thereference value exceeds either or both of them. If the reference valueis less than CLREF or CL, the error signal is computed to be thedifference between the controlled acceleration value (CAREF) and themotor current reading.

The next step is essentially a dead band check, i.e., if the error valueis less than a predetermined minimum, e.g., plus or minus 10 amperes, ajump is made to a later point in the routine. The jump avoids a filtercircuit and prevents oscillations for small errors. If the error isoutside the dead band, a determination of error polarity is necessary.The polarity check (ERROR FLAG NEGATIVE) directs a complement ofnegative errors (the actual value is in two's complement form) in orderto compute a new motor current reference value equal to the old valueminus the error. Positive errors are added directly to the old value toobtain the new value. A digital filter smooths the computed new motorcurrent reference value to prevent step-function changes.

ERROR FLAG NEGATIVE is used again for the purpose of determining thepolarity of the error value in order to determine whether to incrementor decrement the old reference value. For a positive error, if the newreference value is less than or equal to the old reference value, theold value is incremented and saved as the new value. If the newreference value is greater than the old value, the new value issubstituted for the old value. For a negative error, the old value isdecrement if the new reference value is equal or greater than the oldvalue. Otherwise, the new reference value is substituted for and becomesthe old value.

Before applying the new reference motor current value to control thechopper circuit 14, this value is compared with the voltage limit valueestablished by the accelerator pedal position. Referring to FIG. 11 ifthe new current reference value exceeds the accelerator position value,the new value is set at the accelerator position value. Otherwise, thenew value is applied directly to compute the off-time and the on-time ofchopper circuit 14. The computed off-times and on-times are then used togenerate gating signals for the switching devices in the chopper circuit14.

Once the off-time and on-time have been computed, their ratio is thenused to calculate CLREF, i.e., the commutation limit reference describedsupra. Note that the regulator routine is also entered at this point ifthe error value computed earlier had been less than the predeterminedvalue, i.e., the exit at REGLIO enters the routine after calculation ofon-time and off-time. It will be apparent that for small errors, e.g.,less than two amperes, the previously used on and off-times are againsuitable for controlling chopper circuit 14.

After computation of the times for energizing the circuit 14, theroutine checks to determine if this is the first time that the circuit14 has been energized. If an initial turn-on flag is set, the routineexits (REGXIT) to the executive program to perform other functions.Otherwise, the routine continues by setting the tip bounce flag.

Referring to FIG. 12, if the tip bounce timer has timed out, the contacttips are detected to be closed and the tip bounce timer set flag is set,then the routine proceeds to clear the tip bounce and tip bounce timerset flags and to set the chopper 14 initialize flags. The chopper 14 isthen initialized, in FIG. 13, by gating the switches to provide aninitial commutating charge on the commutating capacitor. Charging of thecapacitor is verified before providing signals to gate the main SCRswitch in the chopper into conduction for the previously calculatedon-time to thereby supply power to the motor.

If the flag is set in any of the above checks in FIG. 12 so as toindicate that the monitored condition is negative, the routine exitsback to the executive program momentarily rather than wait for themonitored condition to clear. The routine clears the tip bounce timerset flag before exiting if the tips are closed and the timer flag isset. If the tips are not closed, the tip bounce timer set flag and tipbounce timer flag are set before exiting.

A try-again function is provided in the event that the commutatingcapacitor doesn't indicate a charge after first initialization.

Referring briefly to FIG. 8, it will be recalled that the regulatorroutine early checks to determine whether the system is in a propulsionor an electrical braking, i.e., plugging, mode of operation. Theregulator routine exit at PREG causes a jump to a plugging regulatorsub-routine whose flow diagram is illustrated in FIG. 14. The pluggingfunction is implemented by sensing that the direction switches 118 and120 have been cycled. The logic function is established in the CPU 112such that once the percent on-time for chopper circuit 14 has exceeded apredetermined value, e.g., twelve percent, CPU 112 remembers whichdirection the vehicle is being propelled, i.e., which contactor 104 or106 is energized. If the direction switch is then moved to energize theother contactor, a plug flag is set. A set-up time for plugging isprovided by de-energizing the CPU 112 clock oscillator when thedirection switch passes through neutral.

The regulator function is, to a certain extent, interrupt driven. Theroutine is serviced on a time basis from the executive program but hasselected flag conditions, such as have been described above, whichinhibit regulator operation. In the plug mode one such interrupt is theoutput signal from comparator 138 (see FIG. 7). If the relationshipbetween the relative value of current called for by accelerator 50 andthe actual sensed armature current has not changed since the lastregulator cycle, the present cycle is inhibited. In addition, if thecomparator 138 switches before a predetermined minimum time has elapsedsince the last gating pulse to chopper 14, the interrupt from comparator138 is ignored. This last feature prevents double pulsing, i.e., asituation wherein the first current pulse through chopper 14 forcesmotor current to just below the desired level and the second pulseforces a large overshoot current causing a cogging or jerking effect inelectrical braking.

Although plugging current can be regulated in the same manner asmotoring current, i.e., by varying both the conducting time andnon-conducting time of the chopper 14, the relatively low percenton-times required to maintain a desired braking effort permitsimplification of the control of chopper 14. In the illustratedembodiment, the on-time of chopper 14 has been chosen to be a fixedvalue of 500 microseconds. Motor plugging current is then regulated byvarying the off-time. However, as stated previously, a minimum off-time,e.g., three milliseconds, is provided before another on-time ispermitted. These relative values establish a maximum percent on-time ofabout fourteen percent. Preferrably, plugging is cancelled before themaximum percent on-time is reached. In actual measurements, the onset ofplugging at a relatively high speed might yield a percent on-time as lowas one-half percent. By the time the vehicle slows to about fifteenpercent speed, the percent on-time will have increased to only aboutthree to four percent. However, between fifteen percent speed and zero,the plugging current must increase dramatically to maintain brakingtorque. It, therefore, becomes desirable to terminate plugging when thepercent on-time reaches about twelve percent and to thereafter switch tomotoring mode with its normal current limit.

The detail plugging routine is given in Appendix A. The flow diagram ofFIG. 14 and FIG. 15 illustrate the process described supra. If theinitialization flag is set when the routine is entered, a jump is madeto the calculation steps (PREG1) for establishing and computing currentvalues. Otherwise the initializing values are loaded into the respectivetimers and associated latches. The 500 microsecond on-time is loadedfollowed by loading of a 65 millisecond timer. This latter timerprovides a plug cancel function by keeping track of the total off-time.(For the present system, it is assumed that plug should be cancelled ifthe off-time is less than 3.7 milliseconds, which corresponds to 12percent on-time). The D/A convertor 144 is also initialized at zeroamps.

After initialization, the plug current limit is calculated as a functionof PLPR (plug limit pot reference) and acceleration position. Thecalculated limit is then compared to the last calculated limit value todetermine if there has been a change, i.e., if the accelerated has beenmoved to call for more or less braking effort. If the values are thesame, the plugging exits to REGL20 in the regulator routine (FIG. 11).If the limit values are different, the reference is either increased bya fixed amount or decreased by a fixed amount. The D/A convertor 144 isthen loaded with the new value and the subroutine exited to the normalregulator routine for generation of gating signals and calculation ofoff-times.

Increasing or decreasing the current limit values by fixed amounts tendsto smooth the braking effort to minimize cogging. Exemplary values havebeen chosen to be a five ampere increase and a fifteen ampere decrease.After a decrease, the routine checks to assure that negative currentsare not computed and sets the convertor 144 to zero if negative valuesare detected.

The flow diagram of FIGS. 8-15 in conjunction with Appendix A isbelieved to provide a complete disclosure of a presently preferred formof current regulator in accordance with the present invention. It willbe apparent that various modifications of the microcomputer implementedversion of the invention are possible without departing from the truespirit and scope of the invention. It is therefore intended that theappended claims not be limited to the detailed implementation but coverall such modifications as fall within the spirit and scope of theinvention.

What is claimed is:
 1. A method for controlling an electric tractionmotor propelled vehicle in a plug braking mode of operation, the vehicleincluding a time-ratio power regulator which is controlled to beconducting and non-conducting for connecting and disconnecting the motorto a direct-current power source, an accelerator for establishing adesired value of braking current, sensing means for providing a signalrepresentative of actual motor current, and control means responsive toa comparison of desired braking current to actual motor current forvarying the time ratio of the regulator in a manner to minimize anydifference therebetween, comprising the steps of:(a) sensing that theplug braking mode of operation is commanded; (b) establishing a fixedconducting time interval for the regulator; (c) establishing a minimumnon-conducting time interval for the regulator; and (d) controlling theregulator so that it is alternately conductive for said fixed timeinterval and non-conductive for at least said non-conducting timeinterval, said minimum non-conducting time interval being sustainednotwithstanding any instantaneous differences between the actual anddesired braking currents during said time.
 2. The method of claim 1further including the steps of:cancelling the plug braking mode ofoperation whenever the ratio of the conducting time interval to thenon-conducting time interval exceeds a preselected value.
 3. The methodof claim 1 further including the steps of:(a) repetitively calculating abraking current limit value as a function of accelerator position and apredetermined maximum motor current value; (b) comparing on a cyclicaltime basis the present calculated limit value and the last previouslycalculated limit value; (c) adjusting the present limit value to anadjusted current limit value by incrementing or decrementing the presentvalue by predetermined amounts if the present calculated limit value isgreater than or less than the last previously calculated limit value,respectively; and (d) providing the adjusted current limit value forcomparison to the actual motor current value for controlling the timeratio of the regulator.
 4. The method of claim 3 wherein thepredetermined decrementing amount is greater than the predeterminedincrementing amount.